The present invention relates to a printer of the called laser thermal-transfer type in which ink of an ink sheet is molten by heat energy of focused laser beam, and transferred onto a print sheet for image formation, and an ink sheet used for the printer.
The printer of the called laser thermal-transfer type has been known ("Halftone Color Imaging Laser Dye Transfer", Mitsubishi Nonimpact Printing Technology '93, and Japanese Patent Publication No. Hei. 6-418).
FIG. 14 shows in schematic and block form a conventional printer of the laser thermal-transfer type.
As shown in FIG. 14, an ink sheet 1 is placed on a print sheet 2 on a drum 3. A DC motor 5 is directly coupled with the drum 3. A controller 6 receives a command from a personal computer 4, and issues a command to the drum 3. Upon receipt of the command, the drum 3 is driven to turn with a high precision. With turn of the drum 3, the ink sheet 1 and the print sheet 2 are transported in the transport direction indicated by an arrow Y (referred to the transport direction Y). An optical head 8 is located facing the drum 3. The optical head 8 is supported by a movement stage 9. The movement stage 9 includes a stepping motor (not shown), which moves the optical head 8 in the width directions (indicated by the double arrow X) of the ink sheet 1 (The directions will be (referred to simply as the width direction X.). The stepping motor is driven by a stepping motor drive circuit 10, which receives a command from the controller 6. A semiconductor laser is assembled into the optical head 8. The semiconductor laser is driven by an LD driver 7, which receives a command from the controller 6.
FIG. 15 is a diagram schematically showing the inner structure of an optical head shown in block form in FIG. 14. As shown in FIG. 15, laser beams are emitted from a semiconductor laser 8-1, collimated by a collimator lens 8-2, shaped into a circular beam by a shaping prism 8-3 (since the laser beam emitted from the semiconductor laser 8-1 is elliptical in cross section), focused into a light spot of approximately 10 .mu.m, for example, and projected onto the ink sheet 1.
FIG. 16 is a sectional view showing the structure of the ink sheet. In the structure of the ink sheet 1, a light absorbing layer 1-2 and an ink layer 1-3 are layered on a transparent base film 1-1. A laser beam enters the ink sheet 1 through the light absorbing layer 1-2, and reaches a point on the light absorbing layer 1-2. In the light absorbing layer 1-2, the laser beam is transformed into thermal energy. With the thermal energy, the ink layer is molten. The molten ink is transferred from the ink layer 1-3 to the print sheet 2 that is layered on the ink layer 1-3 of the ink sheet 1. In this way, an image (including characters) is printed on the print sheet 2.
In the printer of the laser thermal-transfer type, one dot in a printed image may be reduced to a size near to the diameter of the focused light spot, e.g., approximately 10 .mu.m. Accordingly, this type of the printer is capable of printing an image of high definition. To produce a printed picture or a formed image of high definition, it is necessary to reduce the beam spot diameter as small as possible. With reduction of the spot diameter, the laser beam must be brought to irradiate exactly at a desired position on the ink sheet 1. To this end, an irregularity or nonuniformity must be minimized in the drum speed of the drum with the ink sheet 1 located thereon. However, it is very difficult to realize such a precise control of the irradiating of the beam spot.
The printing speed of the printer of the laser thermal-transfer type is considerably slower than that of the thermal head type printer since the former transforms optical energy of the light spot into thermal energy to melt ink, while the latter uses a heat generating resistor element. An approach to increase the printing speed of the thermal-transfer type printer is to array a plural number of optical heads in the direction of an arrow X in FIG. 14 and to simultaneously drive these optical heads for printing. A precise control of relatively positioning the laser beam spots, which are emitted from the plural number of the optical heads, on the ink sheet 1 is essential to the approach. In the approach using the plural number of the optical heads, the control of the irradiating positions is very difficult when comparing with that in the thermal-transfer type printer using a single optical head.